A technique for determining an authenticity of a paper sheet based on optical characteristics of the paper sheet has been used in the art. For example, Patent Document 1 discloses an apparatus that detects reflection characteristics and transmission characteristics of a banknote. In this apparatus, two detecting units are arranged opposite to each other across a transport path of the banknote to perform authentication of a paper sheet. Each of the detecting units includes a light emitting unit that emits a light toward the banknote and a detecting sensor. The light is emitted toward the banknote from the light emitting unit of one of the detecting units, a reflected light reflected from the banknote is detected in one detecting unit, and a transmitted light that has passed through the banknote is detected in the other detecting unit. The two detecting units are arranged symmetrically in vertical direction. A reflection image of a front side of the banknote, a reflection image of a backside of the banknote, and a transmission image of the banknote can be acquired by making the two detecting units cooperate. In this apparatus, the light to be emitted can be selected among a visible light, an infrared light, and an ultraviolet light. Therefore, the authenticity of the banknote can be determined by acquiring a feature amount that appears in the reflection image and the transmission image of the banknote depending on the emitted light.
Patent Document 2 discloses an apparatus that emits two types of ultraviolet lights from light sources on a banknote that is being transported on a transport path of the banknote. A light receiving unit in the apparatus receives fluorescent light excited on the banknote because of the light emission. The apparatus acquires a feature amount relating to reflectance characteristics of the banknote corresponding to each of the emitted lights. Two pairs of the light source and the light receiving unit are arranged in this apparatus. Lights having different wavelengths are simultaneously emitted from the light sources, and a reflected light is detected in a corresponding one of the light receiving units thereby acquiring the feature amounts relating to the two types of the emitted lights. Moreover, Patent Document 2 also discloses an apparatus that includes two light sources corresponding to one light receiving unit, and that emits lights having different wavelengths from the light sources but at different timings thereby acquiring the feature amounts relating to the two types of ultraviolet lights.
Among the paper sheets such as a banknote, valuable securities, there are paper sheets in which a technology for preventing counterfeiting thereof, for example, such as a watermark, a hologram, a security thread has been used. Moreover, a paper sheet that is printed with an ink containing fluorescent material or phosphorescent material to prevent counterfeiting thereof is known in the art. When the fluorescent material and the phosphorescent material on the paper sheet are irradiated with an excitation light of a predetermined wavelength band, a fluorescent light and a phosphorescent light are excited. The fluorescent light disappears as soon as the irradiation of the excitation light is stopped; however, the phosphorescent light is emitted for awhile even after stopping the irradiation of the excitation light. The authenticity of the paper sheet can be determined from such a feature relating to an emission characteristic.
Patent Document 3 discloses an apparatus that observes a fluorescent light and a phosphorescent light for determining an authenticity of a paper sheet. In this apparatus, an excitation light of a predetermined wavelength band is emitted on a paper sheet that is placed on a workbench, and the authenticity of the banknote is determined from the observed phosphorescent light. Similarly, Patent Document 4 discloses an apparatus that acquires feature amounts relating to a fluorescent light and a phosphorescent light from a paper sheet. In this apparatus, two line image sensors are arranged at positions shifted in a transport direction of the paper sheet. The fluorescent light is detected with the line image sensor on the upstream side, and the phosphorescent light is detected with the line image sensor on the downstream side.